In the framework of the pilot project “Tumor therapy with heavy ions,” the “Gesellschaft für Schwerionenforschung” [Society for Heavy Ion Research] has irradiated cancer patients having localized tumors in the cranial, neck and pelvic regions since 1997 with carbon ions ranging in energy from 80-430 MeV/u.
The intensity controlled raster scan method, whereby a fine ion beam is conducted in layers over the target volume in a grid array pattern, enables a highly conformal—i.e. adapted to the shape of the tumor—and highly effective irradiation of tumors which are located at some depth in the tissue, while simultaneously protecting the healthy tissue in the vicinity.
A precise, 3-D application dose, however, can only be obtained with a constant predetermined kinetic energy level of the ion beam if the position of the target volume does not change over time. The target volume is frequently referred to as the “clinical target volume (CTV)”. In the cranial region, this can be obtained by immobilizing the head using cranial masks which have been individually fitted. For internal organs however, which, for example, may move during respiration, such as the lungs or organs in the thoracic region, this is not possible. As an example, a movement of the target volume in the chest region is particularly problematic because the target volume may be moved within the “shadow region” of a rib.
The present invention concerns itself with the adjustment of the penetration or range of the ion beam, i.e. the position of the Bragg peak in the irradiated tissue, preferably in a moving target volume. While a lateral shifting, from the perspective of the radiation, of the target volume can be compensated for by a quick control of raster scan magnets, shifts in the direction of the radiation require a quick adjustment of the specific energy of the ions, and thereby the position of the Bragg peak in the depths of the tissue.
This is obtained using a passive, so-called range modulation. A narrow high energy ion beam, of approximately 50-400 MeV/u, and highly focused energy, such as is produced by a synchrotron or a cyclotron accelerator, will experience a well-defined energy loss when passing through a piece of homogenous material having a thickness d. By varying the thickness d of this passive range modulator, also known as a “range shifter,” it is possible to adjust the initial velocity of the ions, and thereby their range in the tissue. The varying of the thickness is obtained through a wedge-shaped, stepped or curved surface structure of the range modulator.
A solution of this sort is already described in the application WO2005/120641, whereby the range modulator therein consists of two wedges which can be slid in opposition to one another. These are mounted on a linear axle driven by an electromotor located directly in front of the patient.
In the present state of technology, the range modulators are generally of a large size, which react insufficiently quickly and are furthermore of a respectively complex mechanical nature resulting in their having costly and production intensive requirements regarding the adjustments to the linear axle drive. In addition, the quick movement of the wedge drives result in significant noise levels, which may be unpleasant for the patient.
Because the range modulators in the present state of technology are also located directly in front of the patient, the ion beams have no precise kinetic energy due to the statically distributed energy loss. Problematically, an expansion of the ion beam as a result of transversal multiple scattering occurs, such that with conventional systems the range modulator must be placed at a minimal distance (typically, approx. 10 cm) from the patient. Furthermore, secondary fragments, such as neutrons, which are generated by nuclear reactions in a range modulator, are not separated from the ion beam, and generate an uncontrolled and undesired additional dose.